Mayor Bloomberg’s controversial $2 billion effort to modernize the 911 system — billed as a cure-all for every emergency-communications ill — was labeled a boondoggle by the city’s own experts two years ago, The Post has learned.

The project “does not have a defined business case” for spending $2 billion on a new 911 system, Gartner Consulting told City Hall in a March 2011 report marked “draft — confidential.”

In bureaucratic-speak, Gartner was telling Hizzoner “the program should not go forward” because the city hadn’t shown a legitimate need, as required by federal guidelines.

The consultant’s 45-page report, reviewed by The Post, explained the city was wasting its money by plowing ahead without resolving key problems. It slams the high-tech system for management failures and computer glitches, and clobbers key communications officials for refusing to cooperate and, instead, battling over turf.

The consultants report also found:

* Repeated failures of the emergency-response software were reported but were not fixed.

* The NYPD refused to merge its system for dispatching units with that of the FDNY and the EMS — although that was a key reason for creating the new system. And the departments would not work together to create a unified management structure for the new system.

* The city agencies involved in the plan would not assist the system’s architects in setting up the new 911 network.

The document has not only been kept from the public but was also withheld from auditors from the City Comptroller’s Office, who spent more than a year analyzing the mammoth project.

“If the city withheld any documents from my office during the course of our audit into the 911 system, they violated the City Charter,” Comptroller John Liu told The Post. “The Bloomberg administration should know by now that it can’t sweep its wasteful projects under the rug.”

The damning assessment of the Emergency Communications Transformation Program echoes much of what was laid out a year later in another document, the 911 Call Processing Review, or 911 CPR.

That report, kept secret for months until The Post revealed its findings, is now the subject of a major lawsuit between city fire unions and City Hall.

A highly edited version of 911 CPR was released in May in response to public pressure. Even the sanitized version of the report said the emergency-communications network was on life support because the technology didn’t work and the Police and Fire departments weren’t cooperating.

At the same time, fire officials started acknowledging that emergency-response times were actually on the rise — instead of decreasing — since the new system was installed.

Bloomberg spokesman John McCarthy said the city stands by its new emergency communications network.

“New York City failed to upgrade its 911 system for decades until the successful overhaul by this administration,” McCarthy said.